Coal fire generation plants use a boiler containing closely placed tubes. For more efficient heat exchange, and power generation, the tubes need to remain relatively clean. Clean tubes allow for better heat exchange across the tubes, which cause more efficient power generation. Additionally, heat transfer tubes require repair and maintenance, due to the extreme nature of the combustion, chemical, and metallurgical processes involved in the production of high pressure, high temperature steam.
During operation of power plants, and particularly coal-fired plants, the heat exchange tubes become coated with slag. Current methods and apparatuses to clean the slag require either high-pressure water canons, explosive charges, or people to enter the boiler and blast the slag from the tubes. In other words, the boiler needs to be shut down and cooled until a maintenance operation can effectively clean the slag from the tubes. The cleaning process can use hydraulic water jets, explosives, and even scrubbing. During the cleaning, the boiler is not operating, and the plant is losing revenue.
It is not unusual for any particular boiler to be shut down for cleaning several times a year. Further, each shut down can last up to 7 or more days. Shutting down a boiler for cleaning can negatively impact a plant's revenue by several million dollars annually.
Although the exact causes of increased slag deposits in boilers are not completely understood, it is believed the lower quality fuel play a role in extended shutdowns. Many boilers were designed for high yield BTU/lb coal, but currently available coal is of less yield BTU/lb. For example, coal from Wyoming's Powder River Basin is rated for a yield of 8500 BTU/lb, which is below the yield most plants were designed for, and has large amounts of contaminants in the form of silicates, minerals, non-combustibles, etc. These vaporize/melt in the combustion zone, and re-condense out on the coolest part of the tubes in the gas stream. This accumulated slag constricts airflow, insulates and damages tubes, and reduces boiler efficiency.
The commonly practiced methods of boiler cleaning and comprise:
1. Hydro-Blasting. A 10,000-psi, 120-130 gpm water jet is delivered thru the access portals with a hand directed water lance. Access is limited to line of sight, and precision is poor. Excessive thermal shocking may damage the adjacent tubes. Effectiveness is good for large slag deposits, but poor for those out of line of sight or captured between tubes. Damage such as tube leaks and tube bending are directly attributable.
2. Explosive Blasting. This process mandates taking the boiler off-line, and inserting explosive charges very near the accumulated slag. It is believed this explosive force can also be fracturing/bending/and damaging the tubes.
3. Load Shedding. Approximately every 48 hours of peak generating (assuming a 350 MW boiler), the plant is throttled to 200 MW for 6+ hours to create a thermal fracturing of the accumulated slag. This is sometimes referred to as thermal cycling. Even with on-line deslagging attempts, it is necessary to also employ the above methods. It is unknown, but suspected, that constant thermal cycling contributes to long-term boiler failures. Thus, it would be desirable to clean the slag from a boiler without the need to shut down and/or cool down the boiler.
4. Use of many installed sootblowers, such as manufactured by Diamond Power International, or Clyde Bergemann, Inc. These devices are generally rail mounted long lances which periodically insert into the boiler cavity for short durations, and blow high pressure steam, air, or water against the waterwalls of the boiler immediately adjacent to their penetration point. They have a major limitation in that they only can clean a relatively small circular area in the immediate vicinity of their penetration point, and have no capability for deployment to other portions of the boiler for cleaning.
For the repair and maintenance of said boilers, it is customary to cease combustion and allow the interior of the boiler to cool for several days whereupon human craft personnel will enter, erect scaffolding, and use traditional methods of metal cutting, grinding, and welding to repair the damaged steam tubes. This process is not only time consuming and tedious, but is also an inherently dangerous activity.